Crossover resistant materials for aqueous organic redox flow batteries

ABSTRACT

An organic flow battery having a positive electrode electrolyte containing organic compounds with extended conjugation and/or cyclic side chains is provided. The flow battery includes a positive electrode and a positive electrode electrolyte including first solvent and a first redox couple. The positive electrode electrolyte flows over and contacting the positive electrode. The first redox couple includes a first organic compound and a reduction product of the first organic compound. The flow battery also includes a negative electrode and a negative electrode electrolyte including a second solvent and a second redox couple. The negative electrode electrolyte flows g over and contacts the positive electrode. Typically, an ion exchange membrane is interposed between the positive electrode and the negative electrode Characteristically, the first organic compound resists crossover through the ion exchange membrane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 62/641,776 filed Mar. 12, 2018, the disclosure of which is herebyincorporated in its entirety by reference herein.

TECHNICAL FIELD

In at least one aspect, the present invention is related to organic flowbatteries with an advantageously diminished amount of crossover.

BACKGROUND

The integration of intermittent energy sources such as solar and windenergy into the electrical grid poses challenges in balancing the supplyand demand. Such challenges can be addressed by employing rechargeablebattery systems capable of storing large-scale electrical energy. Thesebattery systems generally have high energy efficiency and are easilyscalable. Amongst the existing electrochemical storage technologies, theredox flow battery is very promising system for grid scale energystorage. Unlike conventional battery systems, in the flow battery, theredox active material is stored in an external tank as solutions whichcan exist in both reduced or oxidized forms. The power is generated froma cell which is separated from the storage tanks, and can be connected,when needed. Pumps are employed to pass the electrolyte solutionsthrough the cell. The cell typically contains three workingcomponents, 1) positive electrode (cathode) 2) negative electrode(anode) and 3) selective ion conducting polymer electrolyte membrane,separating the positive and negative electrolytes.

One of the principal problems affecting the durability and efficiency ofredox flow batteries is the problem of crossover of molecules from thepositive side of the cell to the negative side and vice versa. Whencrossover occurs, the cell voltage is reduced and the concentration ofthe materials is reduced.

Accordingly, there is a need for improved flow battery design with adecreased amount of crossover.

SUMMARY

In at least on aspect, the present invention solves one or more problemsof the prior art by providing a redox flow battery that resistscrossover from the positive side to the negative side. The flow batteryincludes a positive electrode and a positive electrode electrolyteincluding first solvent and a first redox couple. The positive electrodeelectrolyte flows over and contacts the positive electrode. The firstredox couple includes a first organic compound and a reduction productof the first organic compound. The flow battery also includes a negativeelectrode and a negative electrode electrolyte including a secondsolvent and a second redox couple. The negative electrode electrolyteflows over and contacts the positive electrode. Typically, an ionexchange membrane is interposed between the positive electrode and thenegative electrode. Characteristically, the first organic compoundresists crossover through the ion exchange membrane.

In another aspect, the first solvent and the second solvent eachindependently include water.

In another aspect, the first organic compound and the second organiccompound are dissolved in water and are capable of undergoing reversibleredox reactions under various conditions of pH spanning from acidic toalkaline.

In another aspect, the first solvent and the second solvent are amixture of water and organic solvents may be used.

In still another aspect, the flow batteries have the potential to meetthe demanding requirements for grid-scale electrical energy storage.Other advantages of these battery systems are low cost, durability,environmentally benign and sustainability.

Aspects of the present invention relate to improvements that avoidcrossover and thus achieve high durability and high energy efficiency:addressing the problem of crossover by tailoring the size of themolecules relative to the channels in the membrane; use of membraneswith water content less than 40% to restrict the size of the hydrophilicdomains that facilitate crossover; and use of molecule-membranecombinations where the acidity of the redox molecule is stronger thanthat the acid groups on the membrane. Specifically, the pKa values forthe redox molecule are smaller than the acid groups on the membrane.

In still another aspect, a flow battery that resists crossover from thepositive side to the negative side. The flow battery includes a positiveelectrode a positive electrode electrolyte including water and a firstredox couple. The positive electrode electrolyte flows over and contactsthe positive electrode. The first redox couple includes a first organiccompound and a reduction product of the first organic compound. The flowbattery also includes a negative electrode and a negative electrodeelectrolyte including water and a second redox couple. The negativeelectrode electrolyte flows over and contacts the positive electrode andan ion exchange membrane interposed between the positive electrode andthe negative electrode, wherein the ion exchange membrane impedescrossover therethrough.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 . A schematic of a flow battery system.

FIGS. 2A and 2B. Electrochemical Properties of Biphenol sulfonic acid.

FIG. 3 . Electrochemistry of TMBP and TMBPS.

FIGS. 4A and 4B. Electrochemistry of DMMHQ: 2,2′-methylenebis(benzene-1,4-diol)

FIGS. 5A and 5B. Electrochemistry of DMMS:4,4′-methylenebis(2,5-dihydroxybenzenesulfonic acid).

FIGS. 6A and 6B. Electrochemistry of p-HQSU2

FIG. 7 . Effect of Membrane type on the Capacity of DHDMBS/AQDS cell asa function of the cycling at 100 mA/cm².

FIG. 8 . Comparison the diffusion coefficient of redox active DHDMBSthrough various membranes and their water content.

FIG. 9 . Proton dissociation equilibria and its interaction with theproton dissociation equilibrium redox active molecule.

FIG. 10 . Comparison of the permeation of MMS, BPS, and DHDMBS throughH-PEEK and NAFION.

FIG. 11 . Crossover of various molecules of small size through NAFIONand H-PEEK.

FIG. 12 . Long term stable cycling showing lack of crossover relatedfade in a MMS/AQDS cell with a E750 H-PEEK membrane.

FIG. 13 . NMR of MMS vs AQDS long term cycling showing lack ofcrossover.

DETAILED DESCRIPTION

Reference will now be made in detail to presently preferredcompositions, embodiments and methods of the present invention, whichconstitute the best modes of practicing the invention presently known tothe inventors. The Figures are not necessarily to scale. However, it isto be understood that the disclosed embodiments are merely exemplary ofthe invention that may be embodied in various and alternative forms.Therefore, specific details disclosed herein are not to be interpretedas limiting, but merely as a representative basis for any aspect of theinvention and/or as a representative basis for teaching one skilled inthe art to variously employ the present invention.

Except in the examples, or where otherwise expressly indicated, allnumerical quantities in this description indicating amounts of materialor conditions of reaction and/or use are to be understood as modified bythe word “about” in describing the broadest scope of the invention.Practice within the numerical limits stated is generally preferred.Also, unless expressly stated to the contrary: all R groups (e.g. R_(i)where i is an integer or simply R) include alkyl, lower alkyl, C₁₋₆alkyl, C₆₋₁₀ aryl, C₆₋₁₀ heteroaryl, —NO₂, —NH₂, —N(R′R″)₂, —N(R′R″R′″)₃⁺L⁻, —CF₃, —CCl₃, —CN, —SO₃H, —PO₃H₂, —COOH, —CO₂R′, —COR′, —CHO, —OH,—OR′, —O⁻M⁺, —SO3⁻M⁺, —PO3⁻M⁺, —COO⁻M₊, —CF₂H, CF₂R′, —CFH3, and —CFR′R″where R′, R″, and R′″ are C₁₋₁₀ akyl or C₆₋₁₈ aryl groups; singleletters (e.g., “n” or “o”) are 1, 2, 3, 4,or 5; percent, “parts of,” andratio values are by weight; the term “polymer” includes “oligomer,”“copolymer,” “terpolymer,” and the like; molecular weights provided forany polymers refers to weight average molecular weight unless otherwiseindicated; the description of a group or class of materials as suitableor preferred for a given purpose in connection with the inventionimplies that mixtures of any two or more of the members of the group orclass are equally suitable or preferred; description of constituents inchemical terms refers to the constituents at the time of addition to anycombination specified in the description, and does not necessarilypreclude chemical interactions among the constituents of a mixture oncemixed; the first definition of an acronym or other abbreviation appliesto all subsequent uses herein of the same abbreviation and appliesmutatis mutandis to normal grammatical variations of the initiallydefined abbreviation; and, unless expressly stated to the contrary,measurement of a property is determined by the same technique aspreviously or later referenced for the same property.

It is also to be understood that this invention is not limited to thespecific embodiments and methods described below, as specific componentsand/or conditions may, of course, vary. Furthermore, the terminologyused herein is used only for the purpose of describing particularembodiments of the present invention and is not intended to be limitingin any way.

It must also be noted that, as used in the specification and theappended claims, the singular form “a,” “an,” and “the” comprise pluralreferents unless the context clearly indicates otherwise. For example,reference to a component in the singular is intended to comprise aplurality of components.

The term “comprising” is synonymous with “including,” “having,”“containing,” or “characterized by.” These terms are inclusive andopen-ended and do not exclude additional, unrecited elements or methodsteps.

The phrase “consisting of” excludes any element, step, or ingredient notspecified in the claim. When this phrase appears in a clause of the bodyof a claim, rather than immediately following the preamble, it limitsonly the element set forth in that clause; other elements are notexcluded from the claim as a whole.

The phrase “consisting essentially of” limits the scope of a claim tothe specified materials or steps, plus those that do not materiallyaffect the basic and novel characteristic(s) of the claimed subjectmatter.

With respect to the terms “comprising,” “consisting of,” and “consistingessentially of,” where one of these three terms is used herein, thepresently disclosed and claimed subject matter can include the use ofeither of the other two terms.

It should also be appreciated that integer ranges explicitly include allintervening integers. For example, the integer range 1-10 explicitlyincludes 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Similarly, the range 1 to100 includes 1, 2, 3, 4. . . . 97, 98, 99, 100.

Throughout this application, where publications are referenced, thedisclosures of these publications in their entireties are herebyincorporated by reference into this application to more fully describethe state of the art to which this invention pertains.

As used herein “alkyl” refers to C₁₋₂₀ inclusive, linear (i.e.,“straight-chain”), branched, saturated or at least partially and in somecases fully unsaturated (i.e., alkenyl and alkynyl) hydrocarbon chains,including for example, methyl, ethyl, propyl, isopropyl, butyl,isobutyl, tent-butyl, pentyl, hexyl, octyl, ethenyl, propenyl, butenyl,pentenyl, hexenyl, octenyl, butadienyl, propynyl, butynyl, pentynyl,hexynyl, heptynyl, and allenyl groups. “Branched” refers to an alkylgroup in which a lower alkyl group, such as methyl, ethyl or propyl, isattached to a linear alkyl chain. “Lower alkyl” refers to an alkyl grouphaving 1 to about 8 carbon atoms (i.e., a C1-8 alkyl), e.g., 1, 2, 3, 4,5, 6, 7, or 8 carbon atoms. “Higher alkyl” refers to an alkyl grouphaving about 10 to about 20 carbon atoms, e.g., 10, 11, 12, 13, 14, 15,16, 17, 18, 19, or 20 carbon atoms. The alkyl group can be optionallysubstituted (i.e., a “substituted alkyl”) with another atom orfunctional group such as alkyl, substituted alkyl, halogen, aryl,substituted aryl, alkoxyl, hydroxyl, nitro, amino, alkylamino,dialkylamino, sulfate, mercapto, and the like. In the compounds below,lower alkyl is preferred.

As used herein “aryl” means a monovalent aromatic hydrocarbon having asingle ring (i.e., phenyl) or fused rings (i.e., naphthalene). In arefinement, such aryl groups include from 6 to 12 carbon ring atoms. Inanother refinement, such aryl groups include 6 to 10 carbon ring atoms.Representative aryl groups include, by way of example, phenyl biphenyl,naphthyl, anthranyl, and naphthalene-1-yl, naphthalene-2-yl, and thelike. The term “arylene” means a divalent aryl group.

As used herein “heteroaryl” means a monovalent aromatic group having asingle ring or two fused rings and containing in the ring at least oneheteroatom (typically 1 to 3 heteroatoms) selected from nitrogen, oxygenor sulfur. In a refinement, heteroaryl groups typically contain from 5to 10 total ring atoms. In a refinement, heteroaryl groups have from 6to 16 total ring atoms. In a refinement, the heteroaryl is a C₅₋₁₂heteroaryl. Examples of heteroaryl include, but are not limited to,monovalent species of pyrrole, imidazole, thiazole, oxazole, furan,thiophene, triazole, pyrazole, isoxazole, isothiazole, pyridine,pyrazine, pyridazine, pyrimidine, triazine, indole, benzofuran,benzothiophene, benzimidazole, benzthiazole, quinoline, isoquinoline,quinazoline, quinoxaline and the like, where the point of attachment isat any available carbon or nitrogen ring atom. Additional examples ofheteroaryl groups include, but are not limited to, furanyl, thienyl, andpridinyl group. The term “heteroarylene” means a divalent heteroarylgroup.

Abbreviations:

“AQDS” means anthraquinone-2,7-disulfonic acid.

“BPS” means 4,4′ -dihydroxy-[1,1′ -biphenyl]-3,3′-disulfonic acid.

“DMDHMS” means 2,6-dimethyl-1,4-dihydroxybenzene-3-sulfonic acid.

“DMMHQ” means 2,2′-methylenebis(benzene-1,4-diol).

“DMMS” means 4,4′-methylenebis(2,5-dihydroxybenzenesulfonic acid).

“MMS” means 2,5-dihidroxy-4-methylbenzenesulfonic acid.

“MSE” means mercury/mercurous sulfate electrode.

“PEEK” means polyether ether ketone.

“RDE” means rotating disk electrode.

“TMBP” means 3,3′,5,5′-tetramethyl-[1,1′-biphenyl]-4,4′-diol

“TMBPS” means4,4′-dihydroxy-3,3′,5,5′-tetramethyl-[1,1′-biphenyl]-2,2′-disulfonicacid.

With reference to FIG. 1 , a schematic illustration of a flow batterythat includes a pair of organic redox couples is provided. Flow battery10 includes battery cell 12 which includes positive electrode 14,negative electrode 16, and polymer electrolyte membrane 18. In thecontext of a flow cell, reduction occurs during discharge at thepositive electrode and oxidation occurs during discharge at the negativeelectrode. Conversely, oxidation occurs during charging at the positiveelectrode and reduction occurs during charging at the negativeelectrode. Polymer electrolyte membrane 18 (e.g., an ion exchangemembrane) is interposed between positive electrode 14 and negativeelectrode 16. Positive electrode electrolyte 20 includes water and afirst redox couple 22. In FIG. 1 , a first redox couple 22 is depictedas an example. Positive electrode electrolyte 20 flows over and contactspositive electrode 14. First redox couple 22 includes a first organiccompound Q¹ and a reduction product H₂Q¹ of the first organic compound.During discharge of the flow battery, the first organic compound Q¹ isreduced to the first reduction product H₂Q¹ of the first organiccompound. During charging of the flow battery, the first reductionproduct H₂Q¹ is oxidized to the first organic compound Q¹. Negativeelectrode electrolyte 30 includes water and a second redox couple 32.

Negative electrode electrolyte 30 flows over and contacts the negativeelectrode 16. The second redox couple 32 can include a second organiccompound Q² and a reduction product H₂Q² of the second organic compound.Alternatively, second redox couple 32 can include an inorganic compound.During discharge, the reduction product H₂Q² is oxidized to the secondorganic compound Q². In a refinement, the first organic compound has astandard electrode potential that is at least 0.3 volts higher than astandard electrode potential (e.g., MSE) for the second organiccompound. In a refinement, compounds having a standard electrodepotential that is at least 0.1 V positive to the MSE are suitable forthe positive electrode electrolyte while compounds having a standardelectrode potential that is at least 0.1 V negative to MSE are suitablefor the negative electrode electrolyte. In another refinement, compoundshaving a standard electrode potential that is at least 0.3 V positive tothe MSE are suitable for the positive electrode electrolyte whilecompounds having a standard electrode potential that is at least 0.3 Vnegative to MSE are suitable for the negative electrode electrolyte.Typical operation of the electrolytes is with a solution concentrationfrom 0.1 M to 5 M, solution temperature from 10° C. to 90° C., and pHgreater than or equal to 0 and less than or equal to 14 (e.g., pH 1 to13).

Still referring to FIG. 1 , flow battery 10 further includes a positiveelectrode reservoir 36 in fluid communication with the positiveelectrode 14. The positive electrode electrolyte 20 is stored in thepositive electrode reservoir 36 to charge and discharge the flowbattery. The positive electrode electrolyte 20 cycles through batterycell 12 from positive electrode reservoir 36 via the pumping action ofpump 40. A negative electrode reservoir 38 is in fluid communicationwith the negative electrode 16. The negative electrode electrolyte 30 isstored in the negative electrode reservoir 38 to charge and dischargethe flow battery. The negative electrode electrolyte 30 cycles throughbattery cell 12 from negative electrode reservoir 38 via the pumpingaction of pump 42.

The organic compounds set forth below can be used for either the firstorganic compound or the second organic compound. However, thesecompounds are most advantageously used for the first organic compound.In general, the compounds set forth herein resist crossover from thepositive to the negative side.

In an embodiment, first organic compound Q¹ and/or second organiccompound Q² are redox molecules with extended conjugated systems. Inthis regard, extended conjugation mean that pi (π) system that extendsbeyond and is larger than a phenyl group. Typically, the extended pisystem includes additional pi bonds in groups of 2 or 3 π bonds.Therefore, an extended π system will include from 5 to 12 or more πbonds. Biphenols are a class of redox active molecules that has alsobeen studied in great detail but their redox properties have never beenapplied to the field of organic redox flow batteries. Biphenols are 2electron redox systems. An example of the redox chemistry of biphenolsulfonic acid (BPS) is as follows:

Biphenols have an extended conjugation unlike the quinone molecules thathave been used before in organic redox flow batteries. These moleculesare also readily sulfonated. The electrochemistry of biphenol sulfonicacid using cyclic voltammetry and rotating disk voltammetry shows thatthe charge-transfer kinetics is fast and the redox properties of thismolecule are highly suitable for use in organic redox flow batteries(FIGS. 2A and 2B).

The biphenol molecule is an example of this class of molecules withextended conjugation and also have desirable electrochemical properties.Therefore, variations of the present invention are not only limited tothe biphenolic compounds but is general to any molecule which has anextended conjugation capable of undergoing redox transformations. Theextended conjugation of these molecules are also of larger size whencompared to the one ring quinone system molecules, thus these moleculeswill also be crossover resistant. Examples of such redox activemolecules with substituents that are enhanced in molecular size withextended conjugation is provided by formulae 2-5:

where in m, n, o, p are each independently 0, 1, 2, 3, or 4; q and r areeach independently 0, 1, or 2; s and t are each independently 0, 1, 3,or 4; R₁, R₂, R₃, R₄, R₅, R₆, R₇, R₈, R₉, R₁₀, R₁₁ and R₁₂ are eachindependently —H, —R′, —NO₂, —NH₂, —N(R′R″)₂, —N(R′R″R′″)₃ ⁺L⁻, —CF₃,—CCl₃, —CN, —SO₃H, —PO₃H₂, —COOH, —CO₂R′, —COR′, —CHO, —OH, —OR′, —O⁻M⁺,—SO₃ ⁻M⁺, —PO₃ ⁻M⁺, —COO^(—)M⁺, —CF₂H, —CF₂R′, —CFH₃ and -CFR′R″, whereR′, R″ and R′″ are alkyl or aryl groups. L is any negatively chargedcounter ion (e.g., CL⁻, Br⁻, etc.). M is any positively charged counterion (Na⁺, K⁺, etc.).

As an example of the class of substituted molecules with extendedconjugation, it have been observed that the electrochemistry of themolecules TMBP and TMPBS are well suited from their electrochemicalreversibility as determined by cyclic voltammetric studies (FIG. 3 ).

In another embodiment, the first and/or second organic compounds havecyclic side chains. Using cyclo-alkyl structures as substitutes on theredox system, the molecular size of the redox active molecules can beincreased. These cyclo-alkyl groups are inactive towards redox chemistryand increase the size of the redox active molecule making them lesslikely to crossover across the membrane. Examples of such redox activemolecules with substituents that are enhanced in molecular size withcyclic side chains are provided by Formulas 6 and 7:

wherein i and k are each independently 0, 1, or 2; R₁₃ and R₁₄ are eachindependently —H, —R′, —NO₂, —NH₂, —N(R′R″)₂, —CF₃, —CCl₃, —CN, —SO₃H,—PO₃H₂, —COOH, —CO₂R′, —COR′, —CHO, —OH, —OR′, —O⁻M⁺, —SO₃ ⁻M⁺, —COO⁻M⁺,—CF₂H, —CF₂R′, —CFH₃and —CFR′R″, where R′, R″ and R′″ are alkyl or arylgroups. Y is —(CH2)_(n), —(CH2OCH2)_(n), SO, SO₂, other substitutedC₁₋₁₂ alkyl chains which may contain double bonds or triple bonds. L isany negatively charged counter ion (e.g., CL⁻, Br⁻, etc.). M is anypositively charged counter ion (Na⁺, K⁺, etc.).

In another variation, redox active molecules are formed by linking twoquinone containing moieties. Since small single ring redox activequinone molecules are prone to crossover, increasing their size bylinking two moieties is a way to avoid crossover. Examples of this classof redox active molecules formed by linking two quinone containingmolecules are provided by Formulas 8 and 9:

wherein a and b are each independently 0, 1, 2, or 3; R₁₅, R₁₆, R₇ andR₁₈ are each independently —H, —R′, —NO₂—NH₂, —N(R′R″)₂, —N(R′R″R′″)₃⁺L⁻, —CF₃, —CCl₃, —CN, —SO₃H, —PO₃H₂, —COOH, —CO₂R′, —COR′,—CHO, —OH,—OR′, —O⁻M⁺, —SO₃ ⁻M⁺, —COO⁻M⁺, —CF₂H, —CF₂R′, —CFH₃and where —CFR′R″,where R′, R″ and R′″ are alkyl or aryl groups. X is —(CH₂)_(n),—(CH₂OCH₂)_(n), SO, SO₂, other substituted C₁₋₁₂ alkyl chains which maycontain double bonds or triple bonds. L is any negatively chargedcounter ion. M is any positively charged counter ion. Examples ofcompounds having formula 8 and 9 have been shown to have good redoxproperties based on their cyclic voltammetric behavior and linear sweepvoltammetry at the rotating disk electrode (FIGS. 4A, 4B, 5A, 5B, 6A and6B). The currents indicate that four electrons can be transferred toeach of these molecules.

In another embodiment, membranes with reduced crossover are provided. Inthis embodiment, the flow battery is of the general design of FIG. 1 .Therefore, the flow battery 10 includes a positive electrode14 and apositive electrode electrolyte 20 including water and a first redoxcouple 22. The positive electrode electrolyte 20 flows over and contactsthe positive electrode 14. The first redox couple 22 includes a firstorganic compound Q¹ and a reduction product of the first organiccompound H₂Q¹. The flow battery 10 also includes a negative electrode 16and a negative electrode electrolyte 30 including water and a secondredox couple. The negative electrode electrolyte 30 flows over andcontacts the negative electrode 16 and an ion exchange membrane 18interposed between the positive electrode 14 and the negative electrode16. Characteristically, the ion exchange membrane 18 impedes crossovertherethrough. In this regard, membranes with low water content will havesmaller domains of water in the membrane and will not allow crossover ofthe molecules to occur. In a refinement, the water content is less than40 weight percent of the total weight of the membrane with about 10weight percent being optimal. This class of membranes is represented bymembranes with acid equivalent weights greater than 1100 g/mole ofprotons. In a refinement, the membranes have acid equivalent weightsgreater than 1100 g/mole of protons less than 2500 g/mole of protonsExamples of such membranes includes, but are not limited to,perfluorohydrocarbon membranes, sulfonated hydrocarbon membranes with anarene or substituted arene backbone such as sulfonated polyether-etherketone, sulfonated polyethersulfone, and polystyrene-sulfonic acid-basedmembranes. Advantageously, the membranes of this embodiment, can becombined with the positive and negative electrolytes set forth aboveusing the compounds. therein, in a variation, the first redox coupleincludes a first organic compound and a reduction product of the firstorganic compound such that the first organic compound resists crossoverthrough the ion exchange membrane.

The gradual reduction in capacity of dihydroxydimethylbenzoquinonesulfonic acid (DHDMBS) in a redox flow battery has been tested with thethree different types of membranes. It is shown that the F1850(perfluorohydrocarbon membrane with equivalent weight of 1850 g/ mole ofprotons) and a sulfonated polyetheretherketone (H-PEEK) E750 membranehave significantly reduced fade rate compared to the NAFION 117 membranewith an equivalent weight of 1100 g/mole of protons (FIG. 7 ).

The diffusion coefficient of the redox active molecule through the threedifferent types of membranes have been measured. FIG. 8 shows that thediffusion through the low water content membranes is lower than that ofthe membranes with high water content.

In another embodiment, molecules so chosen with the membrane combinationwhere the acidity of the molecule is greater than that of the membranewill not crossover. When molecules are more acidic than the membrane,they will exist in the ionic form in the membrane. A cation exchangemembrane with sulfonate groups will reject such a molecule. However, forthis type of rejection to be effective, the acidity or the tendency ofthe acid membrane to dissociate and produce protons should be less thanthat of the redox active molecule that has the propensity to crossover.In effect, the pKa of the acid groups on the membrane is greater thanthat of the pKa of the redox active molecule. This method of rejectionof molecules from the membrane ensures that even small molecules do notcrossover. The principle of this action is described in FIG. 9 . FIG. 10provides comparison of the permeation of MMS, BPS, and DHDMBS throughH-PEEK and NAFION.

This invention of molecule and membrane combination that preventscrossover is supported by the example data on crossover rates determinedwith NAFION and sulfonated PEEK membranes. Molecules such as MMS(2,5-dihidroxy-4-methylbenzenesulfonic acid) that are small crossoverthrough NAFION, but do not crossover through H-PEEK. Whereas, DHDMBS isa weaker acid compared to H-PEEK and thus crosses over through H-PEEK.On the other hand, BPS is observed to crossover through NAFION, despiteits larger size compared to DHDMBS and MMS.

The acidity of NAFION is higher than that of H-PEEK. MMS is more acidicthan H-PEEK but less acidic than NAFION. Thus, no crossover is observedwith MMS through H-PEEK membranes (FIG. 11 ).

FIG. 12 shows long term cycling of a MMS/AQDS cell with a E750 H-PEEKmembrane showing lack of crossover related capacity fade due to theeffective membrane-molecule combination. The NMR analysis of MMS vs AQDSshown in FIG. 13 shows no crossover over extended cycling.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

1.-10. (canceled)
 11. A flow battery comprising: a positive electrode; a positive electrode electrolyte including water and a first redox couple, the positive electrode electrolyte flowing over and contacting the positive electrode, the first redox couple including a first organic compound and a reduction product of the first organic compound, a negative electrode; and a negative electrode electrolyte including water and a second redox couple, the negative electrode electrolyte flowing over and contacting the positive electrode; and an ion exchange membrane interposed between the positive electrode and the negative electrode, wherein the ion exchange membrane impedes crossover therethrough.
 12. The flow battery of claim 11 wherein the ion exchange membrane has a water content less than about 40 weight percent of the total weight of the ion exchange membrane.
 13. The flow battery of claim 11 wherein the ion exchange membrane has an acid equivalent weights greater than 1100 g/mole of protons.
 14. The flow battery of claim 11 wherein the ion exchange membrane is selected from the group consisting of perfluorohydrocarbon membranes and sulfonated hydrocarbon membranes with an arene or substituted arene backbone backbone.
 15. The flow battery of claim 14 wherein sulfonated hydrocarbon membranes are sulfonated polyether-ether ketone, sulfonated polyethersulfone, or polystyrene-sulfonic acid based membranes.
 16. The flow battery of claim 11 wherein the first redox couple including a first organic compound and a reduction product of the first organic compound such that the first organic compound resists crossover through the ion exchange membrane.
 17. The flow battery of claim 11 wherein the first organic compound has an acidity greater than the acidity of the ion exchange membrane. 